A gas valve shown in FIG. 6 is used for a premix-type burner. According to the operation of such a pneumatic gas valve, in a state in which first and second solenoid valves 1 and 2 are opened, if gas pressure rises at an inlet 5, an air adjustment valve 4 is moved up. Thus, pressure below the air adjustment valve 4 is dropped, so that a gas adjustment valve 3 is moved down, thereby allowing gas to be introduced into the combustion chamber with predetermined gas pressure.
In addition, if gas pressure drops at the inlet 5, the air adjustment valve 4 is moved down. Thus, pressure below the air adjustment valve 4 rises, so that the gas adjustment valve 3 is moved up, thereby allowing gas and air to be introduced into the combustion chamber with a predetermined mixing ratio.
In addition, when air pressure rises, the air adjustment valve 4 is moved down, so pressure below the gas adjustment valve 3 rises. Accordingly, the gas adjustment valve 3 is moved up so that gas pressure may rise at the inlet, so that gas and air are introduced into the combustion chamber with a predetermined mixing ratio.
In contrast, when air pressure drops, the air adjustment valve 4 is moved up, so pressure below the gas adjustment valve 3 drops. Accordingly, the gas adjustment valve 3 is moved down so that gas pressure may drop at the outlet, so that gas and air are introduced into the combustion chamber with a predetermined mixing ratio.
Such a conventional pneumatic gas valve is operated by means of pneumatic pressure generated from a fan.
Accordingly, the pneumatic pressure generated from the fan must be identical to gas pressure fed into the gas valve. Therefore, it is necessary to significantly increase the pneumatic pressure generated from the fan.
In addition, the air fuel ratio at a high output becomes equal to the air fuel ratio at a low output. That is, it is difficult to adjust the air fuel ratio according to the output even if it is necessary to reduce the air fuel ratio at the high output for improving the combustion efficiency and to increase the air fuel ratio at the low output for preventing erosion of a heat exchanger caused by condensation of exhaust gas.
In addition, the structure of the conventional pneumatic gas valve is complicated, so that the manufacturing cost thereof may be increased.